Hi,

I have a blog which gets daily 2k-3k visits regularly. Its going to be a lot more since I am doing heavy seo for more keywords.

I get very low cpc in adsense. For 150 clicks I get only $10. 90% of the traffic is from US.

I want to sell banner spaces but not sure how much I should charge...

Alot of guys as you know charge per impression which is say $1 for 1k impressions.
The maybe have 3 spots... this is pretty normal.

You should really find a few banner marketplaces and get more of a feel for things.

*TOP TIP* Look through alot of listings on flippa, you will see how many people monetized their sites, and it will give you an idea of what you can charge also for yours.